SEOUL North Korea fired several artillery rounds into the sea near a disputed maritime border with South Korea on Saturday, causing alarm among residents on a nearby island in the South, but the shells did not fly across the border, the South's military said.

The two Koreas are in a tense political standoff after the North's nuclear test in January and a long-range rocket launch on Feb. 7. South Korea and the United States say they were grave violations of U.N. Security Council resolutions.

The allies are expected to begin large-scale annual military drills in early March, which the North calls preparations for war and routinely vows retaliation over.

Saturday's firing, heard from the island of Baengnyeong, was probably aimed in a northwestern direction from the North's shore as part of an exercise, a South Korean defence ministry official said by telephone, asking not to be named.

The South's office of the Joint Chiefs of Staff later said there were several rounds of artillery fire, revising a comment by the ministry official that there had been a single shot, and there were no other unusual movements by the North's military.

South Korea suspended the operation of a jointly run factory park in the North, closing what had been the last window of regular interaction born out of a summit meeting in 2000, when leaders pledged to work for peace and reconciliation.

The area is near the scene of the sinking of a South Korean navy ship in 2010 that the South blames on the North, although Pyongyang denies any role.

The island of Baengnyeong sits just a few kilometres (miles) from the disputed Northern Limit Line (NLL) border and less than 20 km (12 miles) at its closest point to the North's shore.